The present invention relates generally to a method and apparatus for reducing fouling on permeable membrane surfaces, such as Reverse Osmosis (RO) desalination membranes.
Membrane-based separation processes, such as reverse osmosis and ultra- or nano-filtration, are commonly used in industrial applications, such as: desalination, wastewater treatment, and power generation. The major problems associated with membrane-based separation processes include fouling from organic and inorganic contaminants and high pressure loss, which decrease the efficiency of the filtration, while increasing operation costs.
Localized fouling from mineral deposition and biofilms have been correlated with “dead zones” of low flow and low mixing. This lack of fluid scouring allows biofilms to grow in these regions. The small channel size in a typical commercial spiral-wound RO membrane module (typical channel height ranges from 0.1 to a few mm's), promotes laminar flow, due to the low Reynolds number. Laminar flow minimizes mixing from the spontaneous fluctuation in velocity that would be present in turbulent flow in larger channels and higher velocities. Previous approaches for increasing local fluid mixing in these micro-channels include inserting layers of cylindrically shaped filaments in a ladder-type or net-type micro-mixer arrangement. However, these designs suffer from increased pressure drop along the channel, and from regions of low flow and stagnation directly downstream of the cylindrical insert.
What is needed is a micro-mixer design that increases local fluid mixing within these micro-channels to increase the scouring effect on the membrane surfaces, while minimizing the increase in pressure drop; and, while minimizing regions of stagnant flow directly downstream of the micro-mixer.
Against this background, the present invention was developed.